1. Field of the Invention
The present invention relates to a partial discharge monitoring apparatus and a partial discharge remote monitoring system for monitoring partial discharges occurring in rotating electric machines.
2. Description of the Background Art
Partial discharge monitoring apparatuses for detecting and monitoring partial discharges occurring in rotating electric machines, such as turbine generators, are conventionally known.
One example of such a partial discharge monitoring apparatus is disclosed in Japanese Laid-open Patent Publication No. 2002-71742, in which partial discharges are measured by a single-channel detecting circuit which limits a detecting range by using a narrowband filter to remove noise components existing outside the passband of the narrowband filter.
Another example of a partial discharge monitoring apparatus is proposed in Japanese Patent Publication No. 3187642. The partial discharge monitoring apparatus of this Patent Publication employs a first narrowband filter and a second narrowband filter to discriminate between partial discharges and noise components.
Still another example of a partial discharge monitoring apparatus is shown in Japanese Patent Publication No. 2751834. The partial discharge monitoring apparatus of this Patent Publication judges anomalies of a high-voltage device by comparing bandwidths of two specific frequencies.
However, the apparatus shown in Japanese Laid-open Patent Publication No. 2002-71742 has a problem that the apparatus can not remove noise components falling within the passband of the narrowband filter.
Japanese Patent Publication No. 3187642 makes no mention of the remote monitoring capability of the apparatus disclosed therein. As a result of the recent shift to liberalization of power supply, there is a growing need to perform centralized supervisory control of many turbine generators installed at thermal power plants, for example, which are distributed at different local sites. Despite this current trend, Japanese Patent Publication No. 3187642 gives no mention of the remote monitoring capability of the apparatus. In addition, this Patent Publication includes no mention of time resolution needed for collecting signals by using two frequency bands for the purpose of comparison. Typically, several hundred to several thousand partial discharges to be measured occur in a rotating electric machine. A shortcoming of the apparatus of Japanese Patent Publication No. 3187642 is that the apparatus becomes too large and too expensive if this apparatus is to be used for detecting such a large number of partial discharges with high resolution and for transferring data for the purpose of remote monitoring operation.
Further, the apparatus shown in Japanese Patent Publication No. 2751834 has a wide detecting frequency band, so that this apparatus has a problem that it can not remove noise components falling within this frequency band. The approach of this Patent Publication has a shortcoming that the apparatus becomes too large and too expensive.
The apparatuses of Japanese Laid-open Patent Publication No. 2002-71742 and Japanese Patent Publication Nos. 3187642 and 2751834 have some common problems which are explained below.
When a rotating electric machine is in operation, high-frequency noise is generated by the rotating electric machine itself and surrounding devices, and pulses of the high-frequency noise are detected by a partial discharge sensor. According to a two-frequency correlation method in which pulses occurring at random in time are detected by using two frequency bands f1 and f2, high time resolution is needed for measuring the same pulses by using the two frequency bands f1 and f2. To achieve this, each pulse must be detected within a period of 1 μs to 2 μs.
Generally, noise pulses occur in extraordinarily large numbers. If the noise pulses are continuously measured with a time resolution of 2 μs, a maximum of 500,000 pulses will be detected per second by each of detecting circuits having the frequency bands f1 and f2. This results in a total of 1,000,000 pieces of data per second, which would require an enormous memory capacity. In a case where partial discharges occur intermittently, a measurement time of 1 second is insufficient. In this case, a measurement time of about 10 seconds is needed. If pulses are measured with a time resolution of 2 μs, a maximum of 10,000,000 pieces of data will be produced during the 10-second measurement time. This large quantity of data necessitates not only an enormous memory capacity but a long processing time. Again, the apparatus having a capability to handle this quantity of data becomes too large and too expensive.
Another problem is that an extraordinarily long time will be needed for data transfer to enable the remote monitoring operation. To solve this problem, it will be necessary to substantially reduce the number of detected pulses while maintaining the pulse discriminating capability.
One approach to reducing the number of detected pulses would be to set a long peak hold time ranging from 10 μs to 100 μs and to detect a maximum value occurring during this peak hold time. In this approach, however, the detecting circuits having the frequency bands f1 and f2 would detect pulses having different pulse signal intensities, so that two-frequency correlation characteristics are not obtained.